The invention generally relates to digital control apparatus and more particularly to the use of such apparatus in frequency synthesizers.
Prior art frequency synthesizer circuits utilize a read only memory (ROM) which receives a plurality of digital coded input signals and produces a specific plurality of digital coded output signals in response to each different received plurality of coded input signals. The ROM circuits function as table look up devices in which a different specific plurality of output signals is produced for each one of a different plurality of coded input signals applied to the read only memory. Such devices are well known to those skilled in the art and have been used in frequency synthesizer applications.
Generally, the output of the read only memory in a frequency synthesizer consists of a group of binary coded decimal (BCD) signals which are simultaneously coupled to programmable dividers in a phase locked loop while also being coupled to BCD decoders which create drive signals for seven segment visual digital display modules. These seven segment digital display modules are well known to those skilled in the art and are generally driven (excited) by seven individual excitation signals. The BCD decoder drivers for these displays are also well known in the art and perform the function of receiving the BCD output signals from the read only memory and converting these signals into the seven required drive signals needed to drive the seven segment display modules. Thus prior art frequency synthesizers required the use of BCD to seven segment excitation signal decoders for each digital display module. This results in increasing the cost and complexity of prior art frequency synthesizers since logic gates must be provided to perform the necessary decoding function. This is because the programmable dividers commonly available are constructed so as to be driven by one type of digital coded signals (BCD) whereas the seven segment display modules are to be driven by another type of digital coded signals.